Communication compensation system for atmospheric pressure variation



June 8, 1954 L. COMMUNICATION COMPENSATION SYSTEM FOR ATMOSPHERIC PRESSURE VARIATION Filed May 6, 1949 J. ANDERSON 2,680,779

` 31.....'........"'q....' n A A A A l/f/a/fr /gv refr @i if 6 i lNvEN'roR 02m-L Ji i .LESLIE T. ANDEBSCIN a ATTORNEY Patented `une 8, 1954 UNITED STATES ATENT OFFICE COMMUNICATION COMPENSATION SYSTEM FOR ATMOSPHERIC PRESSURE VARIATION Leslie J. Anderson, Moorestown, N. I., assigner to Radio Corporation of America, a corporation of Delaware 1 Claim. 1

their intercom systems or radio receivers decreased, and as their altitude decreased the loudness of the sound produced by the headphones or loudspeakers increased.

An explanation of this phenomenon is that, for a given excursion of the diaphragm of an electro-acoustical transducer, the pressure in the ear canal, which determines, to a great extent, the loudness o the sound heard, is substantially determined by the density of the medium in which the sound is being produced. The density of air is substantiallyy different from ground level up to 40,0% feet and varies quite rapidly with changes in altitude over this range.

In order to compensate for changes in lo-udness with changes in altitude, it has heretofore been necessary to change the gain of the audio amplifiers used in the systems, thereby changing the amplitude of the excursion of the diaphragm of the electro-acoustic transducer. It is readily appreciated that this constitutes a nuisance and distraction to a flight crew upon takeois, landings, o-r other altitude changes, when their attention must be devoted to the many complex tasks involved in modern flying without having to either vary the audio ampliiier gain control or try to hear sound signals which are becoming fainter or louder.

It is therefore an object of my present invention to provide a gain control which automatically compensates for changes in altitude.

It is a further object of my present invention to provide an adjustable gain control for audio ampliners which automatically compensates continuously for changes in altitude or air pressure.

It is still a further object or" my present invention to provide means to maintain the output heard from a sound signal amplifying system substantially constant with changes in altitude.

These and other objects are achieved, in accordance with my present invention, by means of an atmospheric pressure responsive device having a movable element responsive to the ambient pressure, and a miniature type electronic variable f resistance tube the impedance ofthe space path 1 or which is varied by the movement of a control lll electrode which is coupled to the aforesaid movable element. A variable grid bias voltage is derived from the electronic variable resistance tube and is impressed upon one of the early stages 'of a sound amplifying system as a supplementary control bias to vary the output so that the signal heard from the electro-acoustic transducer being driven by the ampliiier is maintained substantially constant with changes in air pressure.

The novel features of the invention as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, when read in connection with the accompanying drawing in which:

Fig. l shows a curve 0f pressure in pounds per square inch versus height in feet,

Fig. 2 shows curves of (l) loss in ear sensitivity with increase in altitude, and (2) the required gain in amplitude of audio signals with increase in altitude to compensate for the loss in ear sensitivity, v

Fig. 3 shows in cross section one embodiment of my invention for generating a voltage the amplitude of which is dependent upon the external air pressure,

Fig. 4 shows a schematic of a typical application of my invention for controlling the output of an intercommunication system in accordance with the altitude,

Fig. 5 shows a typical curve of the variation in resistance of the electron tube of the type shown in Fig. 3 with changes in altitude, the curve having been obtained from an embodiment of my invention which was built, and

Fig. 6 shows a typical curve of the variation of the output voltage available for control bias with changes in altitude, this curve having also been obtained from an embodiment of my invention which was built.

Referring more particularly to the drawing, there is shown, in Fig. 1, a curve i El of the change in pressure in pounds per square inch with the increase in altitude from ground level up to 40,000 feet. The pressure at ground level is substantially 14.7 pounds per square inch and as the altitude increases, this pressure substantially decreases until, at 40,000 feet, it is 2.8y pounds per square inch. The change in pressure thereafter is at a much slower rate.

In Fig. 2 is shown a curve l2 of the loss in hearing sensitivity in the ear as the altitude above ground is increased. Also shown is vthe curve I4 for the gain in the amplitude of audio signals which is mounted a conical diaphragm which moves in response to a pressure diirerentiai between the air pressure Pi in the interior and that on the outside P. The pressure Pi in the interior or the chamber is set at 14.7 pounds, or sea level pressure. A rod 22 attached to the apex 2li of the conical diaphragm 20 extends toward the interior of the chamber and is movable with the diaphragm 20. A bracket 2S is mounted on a wall i8 of the chamber and rigidly supports a miniature electronic tube 2B having a metallic casing n of elongated tubular shape as shown, and a flexible, metallic diaphragm 32 forming a part or the top casing structure. This flexible metallic diaphragm 32 supports, at its center, a rod-like electrode element Sl-l which extends inwardly from the diaphragm and has an external, rodlike extension 35 which is mechanically connected with the rod 22. The connection oi the electrode extension 30 with the rod 22 is at a point in spaced relation to the electrode axis of rotation (in the plane of the diaphragm 32) whereby a desired degree of movement is imparted to the electrode proportionately to the movement or the diaphragm. rlhis movement is in a direction to cause a variation in the interelectrode space between the movable electrode Sri and a second, iixed electrode 38 which is insulated from the casing 30 and has an insulated, external connection lead extending therefrom as indicated. The movable electrode 34 is conductive and is connected with the diaphragm 32 and the casing, both of which are metallic or conductive and thus provide an electrical connection to the movable electrode. The connections it to the movable and the stationary electrodes, as well as to the filament of the tube 23 are made through the chamber wall using an air tight pressure seal 42.

The miniature tube 28 may be of the type shown, described, and claimed in the copending application of Harry F. Olson, Ser. No. 705,967, led October 31, i946, for Electronic Transducers, now Patent No. 2,491,399, issued December 13, 1949. As pointed out therein, the control electrode Sii is supported and fulcrumed at the diaphragm 32. The rod extension 35 and the portion of the movable electrode 34 Within the tube 2t are of such dimensions that relatively small forces applied by the rod 22 to the extension 30 produce displacements of the electrode 34 which are of measurable and significant magnitudes. The tube 28 therefore provides, in effect, an electronic variable resistance device.

In Fig. li, there is shown a system employing a transmitter, such as a microphone d4 used in an intercommunication system, which impresses audio signals on an input transformer 4S. The output from the transformer i6 is impressed on the control grid t4 of a rst audio amplier tube 4t, the output from that stage is applied to a second ampliiier tube 50, and the output from the second amplifier tube is applied to an oui'- put transformer 52. The output from the transformer 52 is applied to a receiver, such as the pair of headphones 54 shown.

Thus far there has been described a typical intercommunication system comprising a microphone, an audio amplifier and a pair oi headphones. 1n order to compensate for changes in altitude, the pressure sensitive, electronic, variable resistance tube 23 has applied across its electrodes a source of D.-C. bias 5t in series with a variable resistance 53. That end of the secondary winding of the input transformer which is not connected to the grid of the lrst audio amplier is connected to one end of the variable resistance 5t. The other end of the variable resistance is connected to ground or to some other suitable point of constant potential. The cathode of the rst audio amplification stage is connected to a source of grid bias ti). A condenser t2 connected in parallel with the variable resistance is for the purpose of lay-passing any audio signal which might otherwise be set up across the resistance 53.

The grid bias 00 and the variable resistance 5B are both set at sea level to a value where the audio signal provided by headphones Eifl has the desired intensity. is the altitude ci the airplane or other aircraft in which this intercommunication system is located is increased, the pressure of the atmosphere is correspondingly decreased, as shown by the curve l0 of Fig. l. The pressure P1 inside of the enclosed chamber which was sealed at sea level therefore forces the conical diaphragm 20 outwardly causing it to move in proportion to the differences in pressures. This causes the electrode 3ft of the electronic device to move proportionately closer to the fixed electrode 35. In turn, this produces decreased interelectrode resistance in the tube 20, and an increased current flows across the variable resistance The grid bias applied to the grid E@ of the iirst audio amplifier tube t8 is thereby increased causing it to go more positive and thus increasing the amplication of the tube The amplitude of the signal applied to the headphones 55 will consequently be similarly increased.

As the aircraft begins to descend, the increasing pressure P on the outside of the chamber produces a reduction in the pressure differential Which causes the diaphragm 20 to move into the chamber, thus causing the movable electrode Slt to move away from the fixed electrode 38 with a consequent decrease in the voltage Ei set up across the variable resistance Therefore, the gain of the amplifier tube dit is automatically decreased with a decrease in altitude. The system thus acts to continuously automatically compensate for the change in sensitivity in the ear with changes in altitude by correspondingly varying the gain of the amplifier.

A typical embodiment of my invention was built using the schematic shown in Fig. 4. A voltage or 150 volts D. C. was applied across the electronic variable resistance tube 2S and a variable resistance or" approximately ohms was used. Measurements were taken of the changes in resistance across the electronic variable resistance tube 2B and also of the voltage Ei developed across the variable resistance with changes in altitude. The curve 55 in Fig. 5 shows that the resistance of the variable resistance tube 23 varied from approximately 50,000 ohms at seal level to approximately 10,000 ohms at an Aaltitude or 40,000 feet.

rFhe curve bis in Fig. 6 shows that the voltage available for control purposes, using components having the above designated values, ranges from approximately three volts at sea level to 15 volts at 40,000 feet. It may also be seen that the curve 66 is substantially similar to the curve I4 which shows the required increase in compensatory amplication.

Using a 6D6 vacuum tube as the tube 48, with a ixed grid bias of approximately 15 volts and approximately 100 volts D. C. applied to the plate, and using the circuit shown in Fig. 4, the variable resistance 58 was adjusted so that sound of suitable amplitude was supplied by the headphones 54 at sea level. Measurement was then taken of the resultant bias voltage Eg available at the control grid, and it Was found that, at sea level, the bias voltage was minus one volt and at 40,000 feet it was increased to minus 13 volts, as shown by the curve 68 in Fig. 7. The resultant volume of sound heard in the headphones over the range of altitude remained substantially constant.

From the foregoing description, it will be readily apparent that I have provided an improved system or apparatus for continuously compensating for the loss in sensitivity of the ears to the sound received from an electrical transducer with changes in the density or pressure of the air. Although I have shown and described but a single embodiment of my present invention, it should be apparent that many changes may be made in the particular embodiment herein disclosed, and that many other embodiments are possible, all within the spirit and scope of my invention. For example, the control voltage produced With the pressure sensitive apparatus of my present invention may be applied to other stages of an audio amplifier or preamplifier besides the input stage, in order to obtain desired results. Furthermore, it may be pointed out that, in addition to being useful to compensate for changes in altitude incident to travel in an airplane, my present invention has use in other applications, as in compensating for the effects of changes in air pressure upon descending into mines or caves, in compression chambers, diving, etc. Therefore, I desire that the foregoing description shall be taken as illustrative and not as limiting.

What is claimed is:

Apparatus to derive a control voltage dependent upon altitude comprising an enclosed chamber having a movable diaphragm as part of its Wall structure, said diaphragm being exposed to and movable in response to atmospheric pressures, the remainder of said chamber Walls being sufficiently rigid to withstand changes in ambient pressure, a variable resistance tube mounted inside said chamber, said variable resistance tube having a movable control electrode therein for varying the space path impedance thereof, said movable electrode extending externally of said tube and being mechanically coupled to said diaphragm to be movable in unison therewith, means including a variable resistance to derive a voltage from said variable resistance tube having an amplitude dependent upon the space path impedance thereof, an audio ampliiier having amplitude control means, a source of audio signals coupled to said amplier, an electro-acoustic transducer coupled to the output of said audio amplifier, and means to impress said voltage upon said amplitude control means whereby the amplitude of the signals heard from said electroacoustic transducer is continuously maintained substantially constant with changes in altitude.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,887,739 Mott-Smith Nov. 15, 1939 2,142,857 McArthur Jan. 3, 1939 2,155,419 Gunn Apr. 25, 1939 2,231,570 Ryder Feb. 11, 1941 2,378,227 Lee June 12, 1945 2,392,218 Anderson Jan. 1, 1946 2,491,390 Olson Dec. 13, 1949 

